1. Field of the Invention
The present invention relates to a viscoelasticity improving agent for rubber and a rubber composition containing the same. More particularly, the present invention relates to a viscoelasticity improving agent for rubber obtained by reacting an alkyllithium with a free radical-containing compound having an isocyanate group in the molecule thereof and stable under an ordinary temperature and in the presence of oxygen and also to a rubber composition containing the same. The preferable rubber composition of the present invention has low heat buildup and a good tangent delta (tan xcex4) balance.
2. Description of the Related Art
A free radical-containing compound (i.e., sometimes xe2x80x9cfree radical compoundxe2x80x9d) which exist stably at room temperature in the presence of oxygen is already known. For example, Japanese Unexamined Patent Publication (Kokai) No. 8-239510 discloses use of a stable free radical to inhibit deterioration and stabilize a polymer by a mechanism not involving oxygen. Japanese Unexamined Patent Publication (Kokai) No. 4-328113 discloses a rubber composition containing a synthetic rubber obtained using an organic lithium polymerization initiator, of which Li ends are trapped with an isocyanate.
Japanese Unexamined Patent Publication (Kokai) No. 10-182881, which is based upon an application filed previously by the inventors, discloses a rubber composition obtained by blending a free radical-containing compound containing a nitroxide radical, hydrazyl radical, allyloxyl radical, and/or trytyl radical having a strong radical trapping function in the rubber composition and providing superior processability, high grip, and low heat buildup, protected from oxidation deterioration at the time of abrasion, and improved in abrasion resistance. Further, Japanese Unexamined Patent Publication (Kokai) No. 11-43557 discloses a rubber composition obtained by blending a rubber with a compound containing a radical having an amino group, isocyanate group, hydroxyl group, carboxyl group, oxirane group, and/or thiirane group such as a nitroxide radical, hydrazyl radical, allyloxyl radical, and/or trytyl radical and existing stably at room temperature in the presence of oxygen, having a superior processability, improved abrasion resistance, and a superior balance of high wet braking performance and rolling resistance (tan xcex4 balance). Further, Japanese Patent Application No. 11-57101 discloses a rubber such as rubber stabilized with a free radical compound such as a nitroxide, hydrazyl, allyloxyl radical, trytyl radical, etc. containing an iminium salt.
For example, a stable free radical compound such as 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) is able to quickly trap radicals formed by cleavage of rubber by light, heat, or mechanical action, and therefore is known to improve the processability. However, it was found that the trapping of the radicals increases the number of ends of the rubber and tends to decrease the physical properties of the rubber.
Various attempts have been made to improve the properties of products made from rubber compositions, in particular tires. The assignee disclosed rubber compositions containing radical compounds as rubber compositions superior in processability and giving a high abrasion resistance and superior balance between high wet braking performance and low rolling resistance when made into tires in Japanese Unexamined Patent Publication (Kokai) No. 10-282881 and Japanese Unexamined Patent Publication (Kokai) No. 11-43557.
However, there have been increasingly strong demands for reducing fuel consumption in cars derived from global environmental issues and needs for energy saving. Further reduction of the rolling resistance, that is, further reduction of heat buildup, has been sought in rubber compositions.
In the past, rubber compositions using free radical compounds containing radicals such as a nitroxide radical and hydroxide radical (Japanese Unexamined Patent Publication (Kokai) No. 10-182881) or compounds containing free radical compounds having isocyanate groups (Japanese Unexamined Patent Publication (Kokai) No. 11-43557) have been known. On the other hand, rubber compositions using elastomers modified by tin (Japanese Unexamined Patent Publication (Kokai) No. 5-112674, Japanese Unexamined Patent Publication (Kokai) No. 7-18125, Japanese Unexamined Patent Publication (Kokai) No. 10-95873, etc.) have also been known. No rubber composition which improves various physical properties by using these two ingredients has yet been known.
An object of the present invention is to provide a viscoelasticity improving agent for rubber which can suppress the increase in the number of rubber ends due to radical trapping, whereby a decrease in physical properties of rubber is prevented, and also to provide a rubber composition containing the same.
Another object of the present invention is to provide a rubber composition superior in low heat buildup.
Still another object of the present invention is to provide a rubber composition having improved physical properties of rubber by blending a suitable quantity of a free radical compound introducing functional groups capable of reacting with functional groups on the surface of carbon black into a rubber composition containing a Sn coupling rubber with a large ability to produce end radicals at the time of mastication.
In accordance with the present invention, there is provided a viscoelasticity improving agent for rubber obtained by reacting an alkyllithium compound with a free radical compound having at least one isocyanate group in the molecule thereof or an analogous compound and stable at an ordinary temperature in the presence of oxygen.
In accordance with the present invention, there is provided a viscoelasticity improving agent for rubber comprised of a free radical compound having in the molecule thereof a functional group represented by formula (I) and/or formula (II): 
wherein R is a C1 to C20 alkyl group or aryl group and existing stably at room temperature in the presence of oxygen.
In accordance with the present invention, there is further provided a rubber composition comprising 100 parts by weight of at least one starting diene rubber and at least 10 parts by weight of at least one reinforcing agent and at least 0.01 part by weight but less than 1 part by weight of a radical-containing compound containing at least one radical having at least one isocyanate group selected from the group consisting of a nitroxide radical, hydrazyl radical, allyloxyl radical, and trytyl radical and existing stably at room temperature in the presence of oxygen.
In accordance with the present invention, there is still further provided a rubber composition comprising 100 parts by weight of at least one starting Sn coupling diene rubber and 0.01 to 1 part by weight of at least one compound selected from the group consisting of those having a nitroxide radical, hydrazyl radical, allyloxyl radical, and trytyl radical having, in the molecule thereof, at least one substituent of an isocyanate group, amino group, hydroxyl group, carboxyl group, oxirane group, thiirane group, ester group, and amide group and which exists stably at room temperature in the presence of oxygen.
In accordance with the present invention, there is still further provided a rubber composition comprising 100 parts by weight of at least one starting Sn coupling diene rubber and 0.01 to 1 part by weight of a nitroxide radical, hydrazyl radical, allyloxyl radical, and trytyl radical having, in the molecule thereof, an isocyanate group and/or amino group or an analogus compound and existing stably at room temperature in the presence of oxygen.
It must be noted that, as used in the specification and the appended claims, the singular forms xe2x80x9ca,xe2x80x9d xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to xe2x80x9ca free radical compoundxe2x80x9d includes mixtures of free radical compounds.